Time-Domain NMR to Investigate the Confinement of Water in Ultrathin BPH Zeolites

In this work, we investigated the dynamics of water in the pores of ultrathin BPH zeolite (Linde Q) nanosheets at different temperatures using ¹H time-domain NMR. Relaxation times and pore size distribution were determined by fitting the curves obtained using (i) Hahn and (ii) Carr-Purcell-Meiboom-Gill (CPMG) echo trains. Two distinct relaxation components and pore sizes were identified and assigned to water molecules in micro- and mesopores, respectively. The relaxation behavior within each individual pore adheres to a fast exchange regime, while an intermediate one governs the exchange between water molecules in different pores. The ratio of longitudinal and transversal relaxation times (T₁₁/T₂₁) of ∼2 reflects a strong surface interaction for water in zeolite pores featuring a surface relaxivity of 0.15 nm/ms. The confinement of water in the pores delays the rates of solid-liquid phase transitions, giving rise to a hysteresis loop reflecting a higher stability for confined liquid water or amorphous ice. These results highlight the possibility of tuning the separation capacities of zeolites in wet environments at different temperatures.